Methods of preparing settable fluids comprising particle-size distribution-adjusting agents, and associated methods

ABSTRACT

Base cement compositions comprising particle-size distribution-adjusting agents, and methods of using such base cement compositions in subterranean operations and surface applications are provided. An example of a method comprises: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; adjusting the density of the base cement composition on-the-fly with a density modifying agent to provide a density-adjusted cement; activating the density-adjusted cement composition; placing the density-adjusted cement composition in a subterranean formation; and permitting the density-adjusted cement composition to set in the subterranean formation. Another example of a method comprises: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; and adjusting the density of the base cement composition at the job site by variably injecting a density modifier into the base cement composition.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is a continuation-in-part application of U.S. patent application Ser. No. 10/759,678, filed on Jan. 16, 2004, titled “Settable Fluids Comprising Particle-Size Distribution-Adjusting Agents and Methods of Use,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to settable fluid compositions, and more particularly, to settable fluid compositions that comprise particle-size distribution-adjusting agents, and associated methods.

Hydraulic cement compositions are commonly utilized in subterranean operations, particularly in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings, such as casings and liners, are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Hydraulic cement compositions are also used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.

Set-delayed cement compositions are often utilized at a number of job sites in circumstances where an operator finds it desirable to prepare a volume of a cement composition that remains in a pumpable state for a long period of time (e.g., for about two weeks or more), and that can be selectively activated to set into a hard mass at a desired time. For example, in circumstances where large volumes of cement are utilized (such as in offshore platform grouting), the equipment required for mixing and pumping the requisite large volumes of cement composition may be very expensive, and may be difficult to assemble at the desired location. The storage of the requisite amount of dry cement prior to use may be another problem. As another example, the use of a set-delayed cement composition may also be desirable in circumstances where a relatively small volume of cement composition is used, such as a small construction job, for example, or a plugging and squeezing operation performed in the petroleum industry, for instance.

In such circumstances, the cost to transport the cement composition to a job site, and to mix and pump it on location may be undesirable relative to the revenue generated from performing the cementing operation. A job site may include any location above-ground or below-ground for which a cement composition may be suitable as well as the area surrounding such locations. Set-delayed cement compositions may be useful in such circumstances, as they can be prepared at a convenient location, then transported to and stored at a job site until use. At a desired time, the set-delayed cement composition may be mixed with a set activating agent; the resulting mixture may then be placed into a desired location (e.g., into a subterranean formation) and permitted to set therein.

In some conventional formulations, an excessive amount of set-activating agents have been added to the set-delayed cement compositions, thereby “over-activating” the cement composition, after which a retarder is then added to the cement composition, in an attempt to fine-tune the eventual set time of the cement composition. This can be difficult to manage.

Additionally, operations involving conventional set-delayed cement compositions may encounter a number of other difficulties. For example, the cement composition may thicken or gel with time, increasing the cement composition's viscosity, and thus impairing its pumpability. Another difficulty is that the activation process may be quite complicated, as exemplified by operations wherein the cement composition's set-time is first delayed until shortly before use, after which the cement composition is over-activated and again retarded.

Another problem that may occur with some conventional set-delayed cement compositions is that the addition of set-activating agents may cause premature localized setting of the cement, e.g., localized regions within the bulk cement slurry wherein the set-activating agent becomes concentrated, thereby causing premature setting of a portion of the bulk cement. Such premature localized setting of the cement composition may be likely to occur, for instance, when the cement composition is inadequately mixed. Premature localized setting of the cement composition may lead to pumping problems (e.g., hardened cement particles may damage pump impellers), and may also cause problems such as setting of the bulk cement while in storage tanks.

An additional difficulty posed by some conventional set-delayed cement compositions is that the performance of the set-activating agents commonly used to selectively activate the cement compositions may be unpredictable. This may cause premature setting of the cement before placement (e.g., where the activating agent imparts an unexpectedly strong activating effect), or delayed setting of the cement after placement (e.g., where the activating agent imparts an unexpectedly weak activating effect). Both are usually undesirable.

Moreover, conventional set-delayed cement compositions often may be prepared in batch and stored at a central location, rather than being prepared at a job site shortly before use. Typically, if there is a need for density modifications to the slurry at a job site prior to pumping, addition of dry density modifying additives to achieve a desired density would be required, which may be inconvenient and would require additional equipment for the addition and the mixing stages. Furthermore, if multiple job sites needing slurries with different densities are to be supplied cement slurries from a single slurry stored in a central location, current technology requires that density adjustment at each job site be accomplished by addition of different levels and types of dry density modifying additives, which would lower the benefits of using a single storable slurry for multiple cementing jobs. Thus, conventional set-delayed cement compositions may lack the ability or flexibility to tune the density as needed from a single set-delayed slurry used in different wells or in a single wellbore at different depths or a single well with varying fracture gradients. Thus, presently the use of conventional set-delayed cement compositions is limited only to those subterranean formations for which the design density of the set-delayed cement composition matches the required slurry density at a job site. There is a need to increase the flexibility of on-the-fly density modification to enable use of a single cement slurry to service multiple wells or multiple depths in a single well.

SUMMARY OF THE INVENTION

The present invention relates to settable fluid compositions, and more particularly, to settable fluid compositions that comprise particle-size distribution-adjusting agents, and associated methods.

In one embodiment, the present invention provides a method of cementing in a subterranean formation, comprising: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; adjusting the density of the base cement composition on-the-fly with a density modifying agent to provide a density-adjusted cement; activating the density-adjusted cement composition; placing the density-adjusted cement composition in a subterranean formation; and permitting the density-adjusted cement composition to set in the subterranean formation.

In one embodiment, the present invention provides a method of customizing the density of a settable composition for use at a job site comprising: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; and adjusting the density of the base cement composition at the job site by variably injecting a density modifier into the base cement composition.

The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of exemplary embodiments, which follows.

DETAILED DESCRIPTION

The present invention relates to settable fluid compositions, and more particularly, to settable fluid compositions that comprise particle-size distribution-adjusting agents, and associated methods. The settable fluid compositions of the present invention are density-adjusted cement compositions wherein a base cement composition (that comprises a particle size distribution agent) is or has been treated with a density modifying agent. These settlable fluid compositions can be used in any application requiring a settable fluid. One of the many advantages of the present invention is that the base cement composition comprising particle-size distribution-adjusting agents may be prepared in batch with a standard density, and then customized to obtain a cement composition with a density appropriate for a particular application. Moreover, this can be done on-the-fly, which is desirable in many instances.

In some embodiments, the present invention provides methods of cementing in a subterranean formation. An example of such embodiments is a method of cementing in a subterranean formation, comprising: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; adjusting the density of the base cement composition on-the-fly with a density modifying agent to provide a density-adjusted cement composition; activating the density-adjusted cement composition with an activator composition; placing the density-adjusted cement composition in a subterranean formation; and permitting the density-adjusted cement composition to set in the subterranean formation. A “base cement composition”, as that term is used herein, refers to a cement composition of the present invention prior to some density adjustment.

In some embodiments, the present invention provides methods of customizing the density of a settable fluid for use at a job site. An example of such a method comprises: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; and adjusting the density of the base cement composition at the job site by variably injecting a density modifier into the base cement composition.

The base cement compositions used in conjunction with the present invention generally comprise water, a cement, a set retarder, and a particle-size distribution-adjusting agent. Density modifying agents with specific gravities in the range of from about 0.1 to about 10 may be added into the fluid stream while carrying out a pumping operation (e.g. on-the-fly) to adjust the density of the base cement composition, if desired. Optionally, other additives suitable for use in a settable fluid may be added. Density modifying additives may be included in aqueous suspensions or other solutions for improved rheology (e.g., mixability and pumpability).

Generally, the density-adjusted cement compositions of the present invention may have a density in the range of from about 4 to about 25 pounds per gallon. Higher or lower densities may be appropriate depending on the application. In certain exemplary embodiments, the density-adjusted cement compositions of the present invention may have a density in the range of from about 10 to about 25 pounds per gallon.

In certain exemplary embodiments of the present invention, the base cement composition provided may be formulated as a “densified” base cement composition (e.g., formulated with a significantly higher density than that which is calculated to be necessary for its intended use) before the addition of the density modifying agent and activator composition. Such a densified base cement composition may be provided in a variety of ways, such as through the addition of high-density particles, or by formulating the base cement composition with less water than necessary for its intended use. Among other benefits, the employment of a densified base cement composition will facilitate the addition of an activator composition in the form of a dilute solution. For example, if a cement composition having a 16.4 pounds per gallon density is required, a densified base cement composition having a density of, for example, 17 pounds per gallon or higher may be provided and activated with an activator composition diluted with sufficient water to ultimately provide the desired 16.4 pounds per gallon slurry. Among other benefits, the addition of the activator composition in a dilute solution to a densified base cement composition may minimize the possibility of developing localized zones having excessive activator concentration due to inadequate mixing. Examples of suitable density modifying agents that may be added for the purpose of reducing the density of the provided densified base cement composition of the present invention include, but are not limited to, density reducers such as water, gas, low bulk density inorganic materials containing entrapped air such as expanded mica and expanded vermiculite, and microspheres. In some embodiments, the density reducer may have specific gravities in the range of from about 0.1 to about 3.0.

In certain exemplary embodiments, where the density of the provided base cement compositions is to be reduced, microspheres may be directly added to the densified base cement composition. Suitable microspheres that may be utilized in accordance with the present invention include hollow, solid, and porous microspheres. The microspheres may be present in the settable compositions of the present invention in an amount in the range of from about 1% to about 90% by weight of base cement composition. The size of the microspheres present in the base cement composition is in the range of from about 5 microns to about 1000 microns, and can be present in a variety of sizes or in a uniform size. The microspheres may utilize a variety of materials in accordance with the present invention, including, but not limited to, glass, soda lime borosilicate glass, silica, gold, silver, palladium, platinum, polymethylmethacrylate, poly(L-lactic acid), polyacrylic acid, latex, alumina, titania, melamine, dextran, fly ashes as mined or expanded, ceramic, other polymeric materials for example, thermoplastics such as polyethylene, polypropylene, polystyrene, and elastomers such styrene-butadiene random or block polymers, ethylene-propylene-dienemonomer (EPDM), and mixtures thereof. In some embodiments of the settable compositions of the present invention, the microspheres utilized are hollow microspheres. The microspheres may be obtained from any suitable source. Examples of suitable microspheres are fly ash hollow beads commercially available from Halliburton under the tradename SPHERELITE, hollow synthetic glass beads commercially available from 3M Corporation under the tradename SCOTCHLITE, elastomeric hollow beads comprising organic fluids under the tradename EXPANCEL, and expandable polystyrene (EPS grade) beads available from Huntsman Corporation.

Where the base cement compositions of the present invention are to be foamed (e.g., to reduce the density of the base cement composition, or to improve its mechanical properties), the base cement composition may be foamed by direct addition of the gas into the base cement composition. For instance, where the base cement composition is foamed by the direct injection of gas into the composition, the gas utilized can be air or any suitable inert gas, such as nitrogen, or even a mixture of such gases. In certain exemplary embodiments, nitrogen is used. Where foaming is achieved by direct injection of gas, the gas may be present in the composition in an amount sufficient to foam the composition, generally in an amount in the range of from about 0.01% to about 60% by volume of the composition under downhole conditions. The base cement composition may also be foamed by gas generated by a reaction between the cement slurry and an expanding additive present in the base cement composition in particulate form. For example, the composition may be foamed by hydrogen gas generated in situ as the product of a reaction between the slurry and fine aluminum powder present in the base cement composition. To stabilize the foam, surfactants optionally may be added to the base cement composition. Surfactant compositions suitable for use in the present invention are described in U.S. Pat. Nos. 6,063,738 and 6,367,550, the relevant disclosures of which are hereby incorporated herein by reference.

In certain exemplary embodiments of the present invention, the base cement composition provided may be formulated as a “lightened” base cement composition (e.g., formulated with a significantly lower density than that which is calculated to be necessary for its intended use) before the addition of the density modifying agent, for example a densifying agent, and activator composition. Such a lightened base cement composition may be provided in a variety of ways, such as by formulating the base cement composition with more water than necessary for its intended use. Suitable density modifying agents for the purpose of increasing the density of the provided base cement composition of the present invention are densifiers such as iron oxides, manganese oxides, zinc oxide, zirconium oxide, iron carbonate or aqueous solutions of soluble salts such as sodium chloride, calcium chloride, cesium chloride, cesium formate and the like. In some embodiments, the densifiers have specific gravities in the range of from about 3.5 to about 10. Examples of suitable densifying agents are HI-DENSE 3 commercially available from Halliburton, HI-DENSE 4 from Halliburton, and MicroMax™ FF from Halliburton. Densifying agents may be included in the cement compositions of the present invention in an amount up to 100% by weight of dry cement.

The water present in the base cement compositions of the present invention may be from any source provided that it does not contain an excess of compounds that adversely affect other compounds in the base cement composition. For example, a base cement compositions of the present invention can comprise fresh water, salt water (e.g., water containing one or more salts dissolved therein), brine (e.g., saturated salt water), or seawater. The water may be present in an amount sufficient to produce a pumpable slurry. Generally, the water may be present in the base cement compositions of the present invention in an amount in the range of from about 25% to about 150% by weight of cement (“bwoc”) therein. In certain exemplary embodiments, the water may be present in the base cement compositions of the present invention in an amount in the range of from about 40% to about 55% bwoc therein.

Any cements suitable for use in subterranean applications are suitable for use in the present invention. Furthermore, any cements suitable for use in surface applications, e.g., construction cements, are suitable for use in the present invention. In certain exemplary embodiments, the improved cement compositions of the present invention comprise a hydraulic cement. A variety of hydraulic cements are suitable for use including those comprised of calcium, aluminum, silicon, oxygen, and/or sulfur, which set and harden by reaction with water. Such hydraulic cements include, but are not limited to, Portland cements, pozzolana cements, gypsum cements, high alumina content cements, silica cements, and high alkalinity cements.

The base cement compositions of the present invention may further comprise a set retarder. Generally, any set retarder may be used with the base cement compositions of the present invention. In certain exemplary embodiments, the set retarders used in the present invention comprise phosphonic acid derivatives, such as those that are described in U.S. Pat. No. 4,676,832, the relevant disclosure of which is hereby incorporated herein. Commercially available examples of a suitable set retarder include those available from Solutia Corporation of St. Louis, Mo. under the tradename “DEQUEST.” In certain exemplary embodiments of the present invention, a sodium salt of a phosphonic acid commercially available from Solutia Corporation of St. Louis, Mo. under the tradename “DEQUEST 2006” is used. A suitable phosphonic acid based retarder is commercially available from Halliburton under the tradename “MMCR”, micromatrix cement retarder. Generally, the set retarder is present in the base cement compositions of the present invention in an amount in the range of from about 0.1% to about 5% bwoc.

The particle-size distribution-adjusting agents suitable for use in the base cement compositions of the present invention may be any compound that desirably affects the particle-size distribution of the base cement composition by agglomerating particles therein such that the base cement composition's rheology remains desirably stable for a chosen period of time. Even though dispersants affect the particle size distribution by deagglomeration, it is believed that particle size adjusting agents which effect the particle size distribution by agglomeration of fine particles are more suitable in the present invention. Among other benefits, the presence of the particle-size distribution-adjusting agent in the base cement compositions may forestall the onset of gelation for a desired period of time. Accordingly, certain embodiments of the base cement compositions of the present invention are capable of remaining stable in a slurry state for several weeks or more before being activated by the addition of an activator composition. Among other benefits, the presence of the particle-size distribution-adjusting agent in the base cement composition tends to cause smaller particles in the base cement composition to agglomerate, thereby tending to narrow the distribution range of the size of the particles in the base cement composition.

One example of a suitable particle-size distribution-adjusting agent is a cationic polymer. Examples of cationic polymers suitable for use with the present invention include, but are not limited to, cationic polyacrylamides, cationic hydroxyethyl cellulose, poly(dimethyldiallylammonium chloride), and cationic starches. In an exemplary embodiment, the cationic polymer used in the base cement compositions of the present invention is a cationic starch. A commercially available example of a cationic starch is available under the tradename “REDIBOND 5330 A,” from National Starch Co. of Bridgewater, Conn.

Generally, the particle-size distribution-adjusting agent may be present in the base cement compositions in an amount sufficient to adjust the particle-size distribution of the base cement composition to a desired range. More particularly, the particle-size distribution-adjusting agent may be present in the base cement composition in an amount in the range of from about 0.01% to about 4% bwoc. Other amounts may be suitable in some applications.

Optionally, the base cement compositions of the present invention may further comprise a yield stress reducing agent. The use of such yield stress reducing agents may be particularly beneficial in certain exemplary embodiments where a densified base cement compositions is used. Among other benefits, the use of a yield stress reducing agent may facilitate pumping of the densified base cement compositions, inter alia, by reducing the force required to move the densified base cement compositions from a static position. While the present invention is not limited by any particular theory, it is believed that the yield stress reducing agent, inter alia, increases the repulsive force between cement particles, thereby preventing them from approaching each other. An example of a suitable yield stress reducing agent is a sulfonated melamine formaldehyde condensate that is commercially available under the tradename “MELADYNE” from Handy Chemicals, Ltd., of Beachwood, Ohio. Another example of a suitable yield stress reducing agent is a sulfite adduct of an acetone formaldehyde condensate, commercially available from Halliburton Energy Services, Inc., of Duncan, Okla., under the tradename “CFR-3.” Another example of a suitable yield stress reducing agent is a sulfonated naphthalene condensate, commercially available from Halliburton Energy Services, Inc., of Duncan, Okla., under the tradename “CFR-6.” One of ordinary skill in the art, with the benefit of this disclosure, will be able to identify a suitable yield stress reducing agent for a particular application.

Optionally, the base cement compositions of the present invention may further comprise an expanding additive. Where an expanding additive in particulate form is used, aluminum powder, gypsum blends, and deadburned magnesium oxide are preferred. Preferred expanding additives comprising aluminum powder are commercially available under the tradenames “GAS-CHEK®” and “SUPER CBL” from Halliburton Energy Services, Inc., of Duncan, Okla.; a preferred expanding additive comprising a blend containing gypsum is commercially available under the tradename “MICROBOND” from Halliburton Energy Services, Inc., of Duncan, Okla.; and preferred expanding additives comprising deadburned magnesium oxide are commercially available under the tradenames “MICROBOND M” and “MICROBOND HT” from Halliburton Energy Services, Inc., of Duncan, Okla. Such preferred expanding additives are described in commonly-owned U.S. Pat. Nos. 4,304,298; 4,340,427; 4,367,093; 4,450,010; and 4,565,578, the relevant disclosures of which are hereby incorporated herein by reference. One of ordinary skill in the art, with the benefit of this disclosure, will be able to determine the appropriate amount of expanding additive to include in the base cement compositions of the present invention for a particular application.

Additional additives may be added to the base cement compositions of the present invention as deemed appropriate by one skilled in the art with the benefit of this disclosure. Examples of such additives include, inter alia, fluid loss control additives, salts, vitrified shale, fly ash, fumed silica, bentonite, fixed-density weighting agents, and the like. An example of a suitable fluid loss control additive is commercially available from Halliburton Energy Services, Inc., of Duncan, Okla., under the tradename “HALAD® 9.”

To prepare the base cement compositions of the present invention for use, an activator composition of the present invention may be added. The activator compositions of the present invention generally comprise a mixture of at least one alkali or alkaline earth metal hydroxide, and a trialkanolamine. A wide variety of alkali or alkaline earth metal hydroxides are suitable for use in the present invention. In certain exemplary embodiments, the alkali or alkaline earth metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide. A wide variety of trialkanolamines are suitable for use in the present invention. In certain exemplary embodiments, the trialkanolamine is selected from the group consisting of: triethanolamine (“TEA”), tripropanolamine, and triisopropanolamine. Such combinations have been found to provide a synergistic effect, resulting in cement compositions that achieve desirably high compressive strengths at a faster rate than would be achieved had the TEA or alkali metal hydroxide been added individually. In certain exemplary embodiments, the alkali metal hydroxide is sodium hydroxide. Generally, the activator composition may be added to a base cement composition of the present invention in an amount sufficient to enable the cement composition to achieve a desired compressive strength and a desired thickening time. More particularly, the activator composition may be added to the base cement composition in an amount in the range of from about 0.1% to 5% bwoc. Generally, the alkali or alkaline earth metal hydroxide may be present in the activator composition in an amount in the range of from about 50% to about 99.9% by weight. Generally, the trialkanolamine may be present in the activator composition in an amount in the range of from about 0.1% to about 50% by weight.

The activator composition may be added in a variety of ways. For example, the activator composition may be added to the base cement composition while the latter is still in storage. In certain other exemplary embodiments, the activator composition may be variably injected into the base cement composition at the same time that the cement composition is injected into the subterranean formation. Among other benefits, the injection of the activator composition while the cement composition is injected into the formation may assist in minimizing the development within the cement composition of localized regions having a high activator concentration.

One example of injecting density modifying agents or other additives on-the-fly into flowing cement slurry includes connecting fluid suspensions to the suction side of the cementing pumping unit. Another example is to variably inject the suspension or solution of a density modifying additive into the flowing cement slurry stream under pressure using a separate pump. By variably controlling the rate of injection, the amount of density modifying material, and as a consequence the density of the slurry can be precisely controlled. This is particularly useful in cases where the formation penetrated by the well is heterogeneous and exhibits different fracture gradients and hence slurry density has to be closely monitored so that the hydrostatic pressure from the cement slurry does not exceed the fracture gradient of the formation resulting in loss circulation. In fact, the method of on-the-fly density modification allows for a quick response in cases where cement circulation loss is encountered during pumping, at which time the density can be variably adjusted appropriately, for example by lowering the density of the slurry being pumped by increasing the amount of density reducing additive or decreasing the amount of density increasing additive being injected, to stop the loss circulation. The ability to control the cement slurry density by on-the-fly adjustment of slurry density is also important in cementing long strings of vertical casing where significant density variation may be needed from the bottom of casing (for example the shoe area) to the top of the cement column. Examples of mixing systems suitable for on-the-fly adjustment of slurry density include the RCM® II Mixing System and RCM® IIe Mixing System commercially available from Halliburton. Using conventional method of single density slurry could potentially may be inadequate to prevent loss of fluids in the deepest part of the cemented zone and sufficient to exceed the fracture gradient near the top of the cement column.

To facilitate a better understanding of the present invention, the following examples of some exemplary embodiments are given. In no way should such examples be read to limit the scope of the invention.

EXAMPLES

The base cement composition provided in the following examples comprised Class G cement (100% bwoc), SSA-1 (35% bwoc), HALAD-9 (0.27% bwoc), CFR-6 (0.196% bwoc), FDP-C754-04 (1% bwoc), FDP-C662-02 (0.375% bwoc), and water (5.62 gausack); with density (16.2 pound per gallon) and yield (1.45 Cuft/sack). The density and compressive strength of the density-adjusted cement compositions described in the following examples were measured according to API Specification 10B, Twenty-Second Edition, December, 1997.

Example 1

Sample No. 1 comprised the provided base cement composition described above, to which 14% bwoc hollow beads were added. Sample No. 2 comprised the provided base cement composition, to which 60% bwoc Micromax™ was added. Sample No. 3 comprised the provided base cement composition, to which 32% by volume of slurry nitrogen foamer were added. The resulting densities of the density-adjusted cement compositions are set forth in the table below. TABLE 1 Density Additive (pounds per gallon) Sample No. 1 Hollow beads 10.74 Sample No. 2 Micromax ™ 18.0 Sample No. 3 Foamer and nitrogen 11.0

Example 1 demonstrates, inter alia, that we can successfully vary the density and obtain reasonable compressive strength out of the set materials.

Example 2

Sample Nos. 1 and 2 described in the previous example were cured at autoclave under 300° F. and 3000 psi for 72 hours. The base slurry sample comprised Class G cement (100% bwoc), SSA-1 (35% bwoc), HALAD-9 (0.27% bwoc), CFR-6 (0.196% bwoc), FDP-C754-04 (1% bwoc), FDP-C662-02 (0.375% bwoc), and water (5.62 gal/sack); with density (16.2 pound per gallon) and yield (1.45 Cuft/sack). The compressive strengths were measured using standard Tinius Olsen equipment (model no. 398) and are set forth in the table below. TABLE 2 Density Compressive (pound per Strength gallon) (psi) Base Slurry 16.2 9340 Sample No. 1 10.74 1709 Sample No. 2 18.0 7600

Example 2 demonstrates, inter alia, that the density-adjusted cement compositions of the present invention could be prepared from a base cement composition successfully.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood as referring to the power set (the set of all subsets) of the respective range of values, and set forth every range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. 

1. A method of cementing in a subterranean formation, comprising: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; adjusting the density of the base cement composition on-the-fly with a density modifying agent to provide a density-adjusted cement composition; activating the density-adjusted cement composition; placing the density-adjusted cement composition in a subterranean formation; and permitting the density-adjusted cement composition to set in the subterranean formation.
 2. The method of claim 1 wherein the particle-size distribution-adjusting agent is a cationic polymer.
 3. The method of claim 2 wherein the cationic polymer is a cationic polyacrylamide, a cationic hydroxyethyl cellulose, a poly(dimethyldiallylammonium chloride), or a cationic starch.
 4. The method of claim 1 wherein the step of adjusting the density of the base cement composition comprises injecting a density modifier into the base cement composition.
 5. The method of claim 4 wherein the density modifier comprises a densifier selected from the group consisting of iron oxides, manganese oxides, zinc oxide, zirconium oxide, iron carbonate, aqueous solutions of soluble salts, and mixtures thereof.
 6. The method of claim 4 wherein the density modifier comprises a densifier which has a specific gravity in the range of from about 3.5 to about
 10. 7. The method of claim 4 wherein the density modifier comprises a density reducer selected from the group consisting of water, gas, low bulk density inorganic materials containing entrapped air, and microspheres.
 8. The method of claim 4 wherein the density modifier comprises a density reducer which has a specific gravity in the range of from about 0.1 to about
 3. 9. The method of claim 1 wherein the step of activating the cement composition comprises adding an activator composition to the cement composition.
 10. The method of claim 9 wherein the activator composition is a mixture of a trialkanolamine and an alkali or a mixture of a trialkanolamine and an alkaline earth metal hydroxide.
 11. The method of claim 1 wherein the step of activating the cement composition is performed before, during, or after adjusting the density of the base cement composition.
 12. The method of claim 1 wherein the step of adjusting the density on-the-fly of the base cement composition comprises injecting a density modifying additive into the base cement composition such that the hydrostatic pressure of the composition does not exceed a fracture gradient of the subterranean formation.
 13. A method of customizing the density of a base cement composition for use at a job site comprising: providing a base cement composition comprising water, a hydraulic cement, a set retarder, and a particle-size distribution-adjusting agent, the base cement composition having a density; and adjusting the density of the base cement composition at the job site by variably injecting a density modifier into the base cement composition.
 14. The method of claim 13 wherein the particle-size distribution-adjusting agent is a cationic polymer.
 15. The method of claim 14 wherein the cationic polymer is selected from the group consisting of cationic polyacrylamides, cationic hydroxyethyl cellulose, poly(dimethyldiallylammonium chloride), and cationic starches.
 16. The method of claim 13 wherein the step of adjusting the density of the base cement composition at the job site comprises injecting a density modifier into the base cement composition before or during placement of the cement composition within a subterranean formation.
 17. The method of claim 13 wherein the density modifier comprises a densifier selected from the group consisting of iron oxides, manganese oxides, zinc oxide, zirconium oxide, iron carbonate, aqueous solutions of soluble salts, and mixtures thereof.
 18. The method of claim 13 wherein the density modifier comprises a densifier which has a specific gravity in the range of from about 3.5 to about
 10. 19. The method of claim 13 wherein the density modifier comprises a density reducer selected from the group consisting of water, gas, low bulk density inorganic materials containing entrapped air, and microspheres.
 20. The method of claim 13 wherein the density modifier comprises a density reducer which has a specific gravity in the range of from about 0.1 to about
 3. 21. The method of claim 13 wherein the step of adjusting the density of the base cement composition at the job site comprises variably injecting a density modifying additive into the base cement composition in response to hydrostatic pressure of the composition in a subterranean formation.
 22. The method of claim 13 wherein the step of adjusting the density of the base cement composition at the job site comprises variably injecting a density modifying additive into the base cement composition to counter cement circulation loss into a subterranean formation. 